1. Field of the Invention
The present invention relates to an optical waveguide element, and more particularly to an optical waveguide element with a directional coupler.
2. Description of the Related Art
An optical waveguide element is an optical element that performs either optoelectric conversion, or a multiplexing and demultiplexing function of optical signals at a transceiving terminal in an optical communication network. In general, the optical waveguide element includes an under cladding layer, a predetermined pattern of core layer, and an over cladding layer that are sequentially laminated on a silicon or polymer substrate.
The transceiving terminal in the optical communication network includes a light source for generating optical signals, and a photodetector for detecting received optical signals. The light source and the photodetector may be separately installed on the transceiving terminal in the optical communication network, respectively. The light source and the photodetector, which have been mounted on one substrate, may be included in a bi-directional optical transceiving module. In the bi-directional optical transceiving module, the light source and the photodetector transmit optical signals to a communication network through one multiplexer, respectively, or receive optical signals from a communication network. Such multiplexers include an arrayed waveguide grating, a multi-mode interferometer and a directional coupler.
FIG. 1 is a diagram illustrating a first type of conventional directional coupler 10. As shown in FIG. 1, the directional coupler 10 includes a first/second waveguide 11 and 12 located adjacent to each other at a predetermined portion for a mode coupling, an input waveguide 11a for providing an input/output of an optical signals, a dummy waveguide 12a, and a first/second output waveguide 11b and 12b. 
The first waveguide 11 and the second waveguide 12 are located adjacent to each other at a predetermined portion, and linearly extend in parallel to each other, thereby providing mutual coupling of optical signals. According to the length of parallel portions of the first waveguide 11 and the second waveguide 12, an optical signal coupled to an adjacent waveguide can be adjusted up to 100%.
The input waveguide 11a extends beyond a predetermined curved section 11c from one end of the first waveguide 11, and receives an optical signal 19 from or outputs an optical signal 19 to a communication network.
The first output waveguide 11b extends beyond a predetermined curved section 11d from other end of the first waveguide 11. The first output waveguide 11b is connected to a light source (not shown) or a photodetector (not shown).
The dummy waveguide 12a extends beyond a predetermined curved section 12c from one end of the second waveguide 12. It is preferred that an end surface 18 is terminated.
The second output waveguide 12b extends beyond a predetermined curved section 12d from other end of the second waveguide 12. The second output waveguide 12b is connected to a light source (not shown) or a photodetector (not shown).
A construction in which the light source is connected to the first output waveguide 11b of the directional coupler 10 and the photodetector is connected to the second output waveguide 12b will now be described. In this construction, an optical signal input from a communication network is input to the first waveguide 11 via the input waveguide 11a. The optical signals are coupled to the second waveguide 12 while passing through the first waveguide 11. The optical signals pass through the second output waveguide 12b, and then are detected by the photodetector. In this case, some optical signals are not coupled to the second waveguide 12, and progress toward the light source through the first output waveguide 11b. The amount of optical signals transmitted to the light source is called one-way cross-talk.
Optical signals emitted from the light source pass through the first output waveguide 11b, and then are input to the first waveguide 11. The optical signals are not coupled to the second waveguide 12 while passing through the first waveguide 11. The optical signals pass through the input waveguide 11a, and then are transmitted to the communication network. While the optical signals emitted from the light source pass through the first waveguide 11, some optical signals are coupled to the second waveguide 12, and progress toward the dummy waveguide 12a. The optical signals that progress toward the dummy waveguide 12a are reflected by the end surface 18, and then again pass through the dummy waveguide 12a. The reflected optical signals are then input to the second waveguide 12. The optical signals are not coupled to the first waveguide 11 while passing through the second waveguide 12, and progress toward the photodetector via the second output waveguide 12b. The amount of optical signals transmitted to the photodetector is called bi-directional cross-talk (hereinafter, referred to ‘BXT’). The BXT implies a distortion of received signals caused by transmitted signals. Removing optical signals caused by the BXT is an important factor dominating the quality of optical waveguide elements such as a directional coupler.
The structure of a conventional directional coupler that attempts to minimize the amount of BXT is shown in FIG. 2 and FIG. 3.
FIG. 2 is a diagram illustrating a second-type of conventional directional coupler 20. As shown in FIG. 2, the directional coupler 20 includes a first/second waveguide 21 and 22 located adjacent to each other at a predetermined portion for a mode coupling, an input waveguide 21a for providing an input/output of optical signals, a dummy waveguide 22a, and a first/second output waveguide 21b and 22b. The first waveguide 21 and the second waveguide 22 linearly extend in parallel at a predetermined portion, thereby providing mutual coupling of optical signals. The input waveguide 21a, a dummy waveguide 22a, and a first/second output waveguide 21b and 22b extend beyond predetermined curved sections 21c, 22c, 21d and 22d from end portions of the first/second waveguide 21 and 22, respectively. The directional coupler 20 receives optical signals 29 or outputs the optical signals 29 through the input waveguide 21a. 
Also, an end surface 28 of the dummy waveguide 22a is terminated in such a way so as to be inclined with a predetermined angle θb with respect to a longitudinal direction of the dummy waveguide 22a. This is done to minimize the amount of optical signals reflected by the end surface 28 of the dummy waveguide 22a. In general, the end surface 28 of the dummy waveguide 22a is inclined with an angle of 82° with respect to a longitudinal direction of the dummy waveguide 22a. Accordingly, the end surface 28 of the dummy waveguide 22a is inclined with an angle of 8° with respect to a sectional surface perpendicular to the longitudinal direction of the dummy waveguide 22a. 
FIG. 3 is a view illustrating a third-type of conventional directional coupler 30. As shown in FIG. 3, the directional coupler 30 includes a first/second waveguide 31 and 32 located adjacent to each other at a predetermined portion for a mode coupling, an input waveguide 31a for providing an input/output of optical signals, a dummy waveguide 32a, and a first/second output waveguide 31b and 32b. The first waveguide 31 and the second waveguide 32 linearly extends in parallel at a predetermined portion, thereby providing mutual coupling of optical signals. The input waveguide 31a, a dummy waveguide 32a, and a first/second output waveguide 31b and 32b extend beyond predetermined curved sections 31c, 32c, 31d and 32d from end portions of the first/second waveguide 31 and 32, respectively. The directional coupler 30 receives optical signals 39 or outputs the optical signals 39 through the input waveguide 31a. 
Also, in order to minimize the amount of optical signals reflected by an end surface of the dummy waveguide 32a, the directional coupler 30 further includes a curved waveguide 38 which extends from the end surface of the dummy waveguide 32a and has a predetermined curvature r. As the curvature r of the curved waveguide 38 grows smaller, the reflection improvement effect increases, and the size of an optical waveguide element including the directional coupler 30 becomes smaller. However, the reflectivity on a boundary surface 38a between the dummy waveguide 32a and the curved waveguide 38 increases.
In the conventional directional couplers described above, in order to remove optical signals passing through a dummy waveguide, an end surface of the dummy waveguide has been terminated in such a way so as to be inclined or a curved waveguide has extended. However, the optical signals passing through the dummy waveguide do not completely disappear, and some optical signals are reflected, thereby causing a distortion of optical signals in the directional coupler. In addition, the distortion of the optical signals in the directional coupler becomes more and more severe when an error occurs in the course of forming an optical waveguide on an optical waveguide element because the optical signals passing through the dummy waveguide do not completely disappear.